Nephrocalcinosis in Fish – Gross pathology

Nephrocalcinosis (or urolithiasis) in fish is a chronic inflammatory condition of unknown aetiology in which calcium and other minerals precipitate as hydroxyapatite within the distal renal tubules and collecting ducts.

Figure 1. Gross appearance of nephrocalcinosis. The kidney is swollen and grey with an irregular surface and white mineral deposits in the ureters.

The disease usually records low mortality and although food conversion efficiency is probably impaired, the major concern about the condition centres round a reduction in carcase quality at slaughter. In severe cases, the muscle dorsal to the kidney may also be affected.

Figure 2. Gross appearance of nephrocalcinosis. The kidney is swollen and grey with an irregular surface and white mineral deposit in the ureters.

There are some predisposing factors for this condition like high levels of carbon dioxide in water (greater than 10-20 mg/L), magnesium deficiency, selenium toxicity and a diet low in minerals. It has also been reported that chronic arsenic exposure in the diet (14mg arsenic/g) causes nephrocalcinosis in Rainbow trout.

Figure 3. Gross appearance of nephrocalcinosis. The kidney is swollen and grey with an irregular surface and white mineral deposit in the ureters.

Other factors including overcrowding and low water flow may also accentuate nephrocalcinosis; similar to high carbon dioxide levels, these lead to acidosis.

Fish with nephrocalcinosis may appear normal or show signs of abdominal swelling, variable exophthalmia, skin petechiae and haemorrhage particularly at the base of the fins.

The grossly swollen grey kidneys in nephrocalcinosis can sometimes be confused with the chronic renal inflammation seen in diseases such as bacterial kidney disease (BKD), proliferative kidney disease (PKD) or indeed some of the mycotic infections such as Exophiala. Histopathological examination quickly eliminates these differentials.